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SESK: Stuff Everyone Should Know · sesk.info/astro-parallax-reciprocal

A star has a parallax of 0.1 arcsecond. How far away is it?

10 parsecs: distance is 1 divided by the parallax.

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  1. 10 parsecs ✔
  2. 0.1 parsecs
  3. 100 parsecs

Why: Distance in parsecs equals 1 divided by the parallax in arcseconds. A smaller shift means a farther star, so 0.1 arcsecond means 10 parsecs and 0.05 means 20.

Huh, didn't know that: Parallax is defined as half the annual shift seen from opposite sides of Earth's orbit.

The dad joke
Parallax is easy: just look at it from both sides of the Sun.
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References

Answer: 10 parsecs. Distance in parsecs equals 1 divided by the parallax in arcseconds. A smaller shift means a farther star, so 0.1 arcsecond means 10 parsecs and 0.05 means 20.

  1. OpenStax Astronomy 2e, 19.2 Surveying the Stars openstax.org/books/astronomy-2e/pages/19-2-surveying-the-stars
    a star with a parallax of 0.1 arcsecond would be found at a distance of 10 parsecs
    Checked 2026-10-10.

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